462952 Alloy Catalysis Across Composition Space

Tuesday, November 15, 2016: 5:05 PM
Franciscan A (Hilton San Francisco Union Square)
A.J. Gellman, Irem Sen, Gamze Gumuslu, Petro Kondratyuk and Xiaoxiao Yu, Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA

Alloys are known to possess superior catalytic properties than their pure components. Finding the rational design of new alloy catalysts with optimal catalytic properties for a given application is the major challenge in multicomponent catalyst design due to the need to perform many catalyst preparation, characterization and reactivity measurements across composition space. To accelerate this search Composition Spread Alloy Films (CSAFs), thin multicomponent films that have composition gradients parallel to their surfaces, AxByC1-x-y with x = 0 → 1 and y = 0 → 1-x, are prepared to be able to span the entire composition space. Many otherwise intractable fundamental scientific problems in alloy science and catalysis that can be effectively addressed through use of CSAFs as high throughput materials libraries.

Study of alloy catalysis across composition space using a CSAF requires a multichannel reactor system that can be used to run steady state catalytic reactions at many different positions or alloy compositions on the CSAF. We have developed a 100 channel microreactor array that can sample product distributions from 100 different alloy catalysts of about 10 minutes. CuxAuyPd1-x-y CSAFs spanning all of binary and ternary composition space have been prepared using a rotating shadow mask CSAF deposition tool which is designed and developed in our group. CSAF surface composition and valence electron energy distributions are measured using X-ray Photoemission Spectroscopy (XPS).

The relationship between alloy catalyst activity and electronic structure has been investigated experimentally across a broad, continuous span of CuxAuyPd1-x-y composition space. The CSAF was used as a catalyst library with a multichannel microreactor to measure H2-D2 exchange kinetics at 100 discrete compositions on the CSAFs over a temperature range of 333 – 597 K at atmospheric pressure. H2 conversion was chosen to be the indicator of activity. It was found that H2-D2 exchange activity varies across the CSAF and it tends to increase with increasing Pd content. When the activities on AuPd and CuPd binary regions are compared, it was found that more Pd is needed in CuPd than in AuPd to achieve the same activity.  

A microkinetic model that has been validated using a number of single component Cu-Pd catalysts in a fixed bed reactor was used to estimate the energy barriers to dissociative adsorption () and associative desorption () of H2 as functions of alloy composition, x and y. On the CuxAuyPd1-x-y CSAF, increasing Pd content from 0 to 1 was found to decrease adsorption barrier from 0.44 to 0.12 eV. Increasing Pd content from 0.25 to 1 was found to increase desorption barrier from 0.4 to 0.74 eV which suggests H2-D2 exchange reaction is limited by H2 desorption step within this Pd content.

Spatially resolved X-ray photoelectron spectra were obtained from the CSAFs and used to estimate the energy of the valence-band center as a function of alloy composition. The v-band center shifted monotonically from -3.4 to 5.6 eV across the CuxAuyPd1-x-y CSAF. The barrier to dissociative adsorption of H2 was found to decrease as the v-band energy increases. This data provides the first experimental correlation of elementary reaction barriers with valence band energy across a continuous span of alloy composition space.

 


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